There's been a few examples, in part these examples have pushed this more into the spotlight.  It's often under the term 'traffic shaping', if you are interested...here is a company that offers those shaping services https://www.sandvine.com/solutions/traffic-optimization/fair-use-policy.html

If you are interested in the peer2peer domain (which is heavily bringing this issue to the forefront) here is a national list of ISP's and their throttling practices on bit torrent users:  https://wiki.vuze.com/w/Bad_ISPs   Curious if this documented list counts as citing.


For the US:

Verizon has ran testing on it.  https://www.digitaltrends.com/web/verizon-wireless-throttling-video-traffic/

> “YouTube is being throttled to 10Mbps as well,” one person wrote on Thursday. “In the ‘stats for nerds’ it would load at roughly 1,250KBps which translates to 10Mbps. Put the VPN on and that number tripled easily. Didn’t have an issue playing 1080p in 60fps, though.”

(part of the problem with throttling is if you know your way around it, there isn't an issue.  Tax on the non-tech savvy).  

Verizon stated they were not throttling, however articles suggest they did and still are https://www.extremetech.com/computing/186576-verizon-caught-throttling-netflix-traffic-even-after-its-pays-for-more-bandwidth

ANd here is an article stating Verizon is actively throttling video connections over their network : https://arstechnica.com/information-technology/2017/08/verizon-to-start-throttling-all-smartphone-videos-to-480p-or-720p/

Over to comcast who has been engaging in this practice but not admitting it until much more recently:

https://www.techdirt.com/articles/20080213/133855251.shtml

when Comcast throttled bittorrent users, they had a lawsuit brought against them that won.  https://www.cnet.com/news/fcc-formally-rules-comcasts-throttling-of-bittorrent-was-illegal/

Sprint:
They deny they do, though it's apparent they can.  User backlash results in people monitoring if they are being throttled.  http://www.sprintusers.com/forum/showthread.php?t=227529

Of course there are several accusations of it:
https://mashable.com/2012/01/05/truly-unlimited-data-throttle/


Bell and Rogers in Canada.  https://www.itbusiness.ca/news/controversy-over-traffic-throttling-by-canadian-isps-heats-up/3632

Telus has also blocked union sites and have engaged in some pretty underhanded anti-union tactics.  Not sure if blocking sites falls under net neutrality though.