Well Android 11 will be a problem, we currently do not have the support layer "Halium 11" up and working. We are currently struggling with 9 and 10. Of course @edenmutt you are invited to help doing groundbreaking works here, but it might not what you have expected for porting. A lot of low-level bugs, and you need to familiarize yourself with the architecture changes between 9 and 11 at least.
@makeixo I am certainly no expert, but as I have understood the mainline Linux kernel will take some effort to get things like power efficiency to the level that a majority of users will consider "usable".... meaning that even if the hardware is working that getting it optimized enough to match Android performance will take a lot of time, and as @Flohack mentions it may take years to get that mainlined, by which time users will have moved on.
Since this thread started on the Pixel 4a and there was some question about how close it may be to the Pixel 3a and 3a XL that are very affordable right now, and thanks to @fredldotme 's effort has a very high compatibility ranking that this would be a good example of the benefit of continuing in the Halium approach: use the kernel that the device ships with but get Ubuntu Touch to run on top of it. Yes it takes time, but it seems much less than trying to mainline devices not designed to use "proper Linux" from the beginning.
Not all of us have the skills like @fredldotme to get a port to this level of quality, but if there are even a few popular devices of recent builds then I think it is enough to sustain UT: So we have the N5 and OPO as "promoted" devices from that older era, then you could consider OP3, Xperia X, Nexus 6P from that generation, then next gen we have Pixel 3a, maybe others. These are already a few years old, but are new enough they will hold out for a few years, after which OP6, others from a newer generation may be available (newer devices that will need Halium 10+)?