I don't think it will "break" (in terms of already started careers at least), but most likely it'll become not compatible anymore with TU 4 (and with the following ones) so it'd need to be updated by his creator.
When it gets updated for a newer TU, it usually also comes with updated squads... So problem solved (until the following TU).
About mods released for previous versions of the game... It is a matter of luck.
I experienced difficulties with EEP for FIFA 20 for example (I didn't want to use the latest version because it already had 20/21 rosters and kits, but the latest 19/20 version was made for an older version of FIFA 20 which I didn't have anymore) but I don't know what happens with Realism Mod Lite.
As I wrote earlier, I have a TU 3 folder + Realism Mod Lite and a TU 3.1 folder with EEP and both work a charm.
Never tried to apply Realism Mod Lite to FIFA 22 with TU 3.1 but
@FIFER Mods can shed a light on this.