That is absolutely not a blow to ROMs by any means. There are plenty of good alternatives. For messaging, alternatives are even better. Dialer apps are just basic ones buy they do exists.
These apps are open source, so a OS developer should be able to forward port the deprecated app into their newer OS release. Kinda fear monger a bit here
Apparently they've been doing this for a long time, because Google haven't been maintaining it. When the forks have submitted improvements upstream they've been rejected. LineageOS has been running its own dialer for years, which many ROMs use.
I use simple SMS anyway, I honestly would love to see a modern dailer for degoogled android. I wonder if we could get the little verified caller checkmarks on a Foss dailer app