Where are all the Europeans gloating about privacy and their "regulations" while their governments are basically erasing actual privacy and free speech?
As far as I'm aware, they are only required to allow you to use your own router.
DSL tech is far simpler and it's always a combo unit so I could see a case where you would be allowed to bring your own DSL modem.
But it just doesn't work like that for DOCSIS or GPON where the cable modems or ONTs these days do much more than just media conversion - SIP, PPPoE, IGMP, etc. even if they don't do Wi-Fi (so ISPs don't call them "routers" - except SingTel, which uses "ONR" to distinguish these units because they are in fact routers for IPTV and SIP).
For all of those modems/ONTs, the firmware updates and the configuration for telephony/SIP and PPPoE are controlled by the ISP and also tested to work with their OLT or CMTS so it's just not possible for the ISP to guarantee support for any random modem or ONT.
And to support the advanced configuration required these days for VoIP, IPTV, etc. on the "modem" or "ONT", ISPs basically have a backdoor called TR-069 which is really not too dissimilar to what Starlink has access to with their SSH keys.
Even if you get "true" dumb modems or ONTs which do not do any routing whatsoever, the device on the other side still has full control over your dumb device via the DOCSIS provisioning process or GPON's OMCI. Starlink seems to be using SSH instead of building a whole protocol - because satellite tech is proprietary and doesn't need to work on other hardware.
So, I find that it's highly unlikely that the ISP is officially required to support a user supplied modem, although I haven't consulted the EU laws on this.
At most, I think using your own router would require the EU ISPs to provide bridge mode support, but that's not special to EU. However, the TR-069 backdoor is still active even with bridge mode.
It can be fairly easy to stop TR-069 with a "dumb" ONT (usually SFP) but ISPs can and will notice that. Whether they allow it is up to them.