Hi @awouwou,
even if device-specific customROMs have a higher level of detail compatibility than phones with Generic System Images (GSI), it is possible to switch to AOSP GSI with every “new” phone, theoritically from Stock Android 8.1, practically better from Stock Android 9, for which it must also be possible to unlock the bootloader, and thus use the phone Google-free. My Galaxy S9+, released in March 2018, shows a possibility …