Following the suggestions, I have enabled multilib support on 4.8, 5.3 and 6.0. Now, with all the portage trouble over USE flags, it would be best if there were no EAPI<5, non-multilib versions >=4.8. This is not a big issue as long as they're all masked but I'd appreciate if we could decide how to handle this in the future. Possibilities: 1) keep the interim versions masked permanently, 2) bump them to EAPI=5, 3) enable multilib support on all of them and remove non-multilib versions.
the remaining 5.x versions are all EAPI5 now.