after emerge --sync I realised that there is no kde-plasma-5.12.x ebuilds in portage tree. And there is no this packages in kde overlay too. They are still supported in kde as LTS. And what really matter thy are more stable stable comparing to new releases. Could you restore this ebuilds in tree or add it to kde overlay ?
The question is what we can support with our manpower, which is independent of what upstream does. Rolling release is a bad match for LTS ideas.
The dependencies of KDE Plasma in Gentoo are already far ahead from at the time of 5.12 release, which is not what upstream is targeting with LTS. No dev was using 5.12.x at the time of removal. I am not aware of issues with current stable KDE Plasma 5.14.3. Bottom line, keeping it around makes no sense in Gentoo.