(OK, I still think this is a bizarre question to be asking and it sucks to oxygenate Peskov but...)
(Update: I am linking to my later answer, to essentially the same question, which includes links to the OSCE observers. You can judge for yourself how much hardware was being observed prior to Feb 24th).
Is there any proof that Ukraine was planning...
Is there? How come Russia did not provide any or why did this question not include such proof? After all, that would clearly show their point so...
To quote the OP's comment
And this isn't a small claim, and one that could be easily disproved by supplying a few satellite images. Maxar was already taking pictures of the Russian forces, it seemed to me it wouldn't be hard for them to snap a few pics of the Ukrainian side while they're at it.
A "few pics" isn't what we are talking about here. You'd have to show all the Donbass area and, presumably, from both the OP and Peskov, show it to be empty of Ukrainian troops. Which wouldn't happen, because this has been a heavily fortified area since 2014.
If an inch was not shown, it could just mean that Ukraine was hiding in it, by this line of reasoning.
Capabilities aren't symmetrical and Ukraine wasn't expected to be winning
First, in February 2022, Ukraine was a dead duck walking. Russia was going to roll over it pronto. Since the war has been turning out otherwise, it is tempting to say "well, yes, they could have invaded. Russia sucks".
But that's rewriting in hindsight. Ukraine couldn't expect to be doing so well before it started.
Capabilities are also asymmetrical. Russian AFVs moving forward around have been getting ambushed by ATM (anti tank missile) teams. That favors the defense. But not necessarily the offense. Those same ATM teams would suck moving forward on foot towards entrenched infantry (aka Donbass).
Ukraine can't afford it
Right now, Ukraine's fate relies both on the morale of its soldiers and on Western support, whether through sanctions or weaponry deliveries. It would not have gotten any great assistance had it assaulted Donbass. Make no mistake, while the West was thoughtless in proposing NATO membership in 2008 it also wasn't happy being put on the spot in 2014 and 2022. It wouldn't have done anything to help recover Donbass.
What about NATO accession?
This war is, partially, about NATO accession. One thing to remember is that any one country can veto admission. The ongoing Donbass conflict was already a hindrance to Ukraine admission and countries were blocking Ukraine's request: "Oh, I know, let's attack Donbass, that'll convince them to let us in"
Been there, done that.
Now, this has actually happened before. People will tell you Georgia was invaded by Russia in 2008, but the actual sequence of events is that Georgia first tried to recover their separatist territories by force. But sheer coincidence, 40000 Russian troops happened to be there, ready to roll on short notice and punched into Georgia. Less than a week later it was all over. Now, people familiar with military matters will know how difficult it is to fire off 40000 troops on short notice. Unless they're ready to go already.
Ukraine knows this and they certainly would not want to give them this excuse. The Russian military has 800k soldiers. Ukraine 200k. 140M to 40M people, Russian GDP is 9x Ukraine's.
Ukraine had standing orders to avoid returning fire
In the run up to the invasion Ukrainian troops had been told to minimize return fire to avoid provocations. In the real world, there are risks to telling your troops not to attack while in fact planning to attack. Putin not having known that does not make it less true.
This is a government that lies, lies and lies.
- NATO/US has biowarfare labs in Ukraine
- Ukraine is shelling its own people, for example the tragic Mariupol maternity hospital was a false flag operation
- Russia was just on exercises
- The GRU agents who were trying to poison Skripal and ended up killing a poor woman were sightseeing a cathedral they didn't even see, for one day.
- Zelensky, a Jewish man, is really a Nazi.
- There is no war going on, Russia is not even attacking Ukraine.
Not all of these are easily disprovable. How do you prove Ukraine didn't shell that Mariupol maternity ward? You can't, unless you find the Russian troops that know they did it. The Russian gunners serving those, long range, guns most likely don't know themselves, assuming they've even heard of the attack in the first place.
As Goebbels supposedly once said, "repeat a lie often enough and people will believe it". That might partially be true. The trouble is that the Kremlin says so many different lies that it is hard to believe any of what it says. So why give any real thought to something that is so off-the-wall bizarre?
"The US lies as well - look at Iraq WMD"
So? What about it? Assuming that the US did know there were no WMDs that only means the US lied. Not that Russia is telling the truth. The 2 are unrelated.
As I said before in comments, it is one thing to examine certain uncomfortable aspects of Ukraine like the ousting of Yanukovich. Or the presence or not of neo-Nazi elements in Ukraine's parliament, military and public. Others can be added too, like the US involvement, if any in 2014 Maidan movements.
This site has, IMHO, been closing a lot of reasonable answers and questions that put Ukraine in a less than flattering light. News to you: being aggressed does not make you a saint and you can be in the right and still have skeletons in your closet.
But it is another thing entirely to discuss each and every outlandish bit of crap the Kremlin is throwing out to justify its war of aggression. This is not the purpose of this site. And this question shouldn't remain open in that sense.
i.e. why not "investigate" if the Mariupol maternity strike was a false flag attack while we are at it? What's so different?