GAME FREAK considers LEGENDS to be mainline games, as they are developed by GAME FREAK, are canon ꦰto the existing Pokémon game universe, and slot into the current generation they release into. Spin-off isn't a word used to describe these games in any official capacity.

I don't know where people keep getting this from, but I feel pretty confident everyone saying this never played the PS1 version of the game. It has the same free roaming as THPS 4 on other platforms𓂃, just missing the NPCs (goals are activated by interacting with floating goal tokens instead) and with smaller levels.