The law in question is the Counter-Terrorism and Border Security Act 2019, which amongst a number of other things made changes to the Terrorism Act 2000 bringing into force a criminal offence for travelling to or failing to leave particular areas of the world.
The pertinent parts of the Terrorism Act 200 are:
Entering or remaining in designated areas overseas
58BEntering or remaining in a designated area
(1) Subject to subsections (3) and (4), a person commits an offence if—
(a) the person enters, or remains in, a designated area, and
(b) the person is a United Kingdom national, or a United Kingdom resident, at the time of entering the area or at any time during which the person remains there.
A "designated area" means:
(10)In this section—
“designated area” means an area outside the United Kingdom that is for the time being designated for the purposes of this section in regulations under section 58C;
And the referred to 58C says:
Section 58B: designated areas
(1)The Secretary of State may by regulations designate an area outside the United Kingdom as a designated area for the purposes of section 58B if the following condition is met.
The phrase "by regulations" here means through a statutory instrument. In other words, the government can through an SI declare an area to be a "designated area" for the purposes of the Terrorism Act 2000, and thus criminalise travel to that area under that Act. There are however no statutory instruments listed on legislation.gov.uk that are made under the power granted in the Terrorism Act 2000, so I conclude that Syria has not been made a designated area for the purposes of that Act. That website is sometimes out of date, but if it had happened more recently one would expect it to have been in the news.