ZDoom

Is that a (n official) way to make a line block melee attacks? Or better, to make a monster not start its melee sequence when player is on the other side of said line? We have hitscan, sound, projectile, player movement, monster movement blocking, etc... is it possible to make with melee?

If you are talking about a single linedef, you can use Block Everything to block melee included, however if you want to only block melee you can use acs as well. Make a sector next to the wall and use the map spots Actor Leaves Sector and Actor Enters Sector to activate SetActorProperty scripts for APROP_MeleeRange of the things behind the wall.

Monster melee attacks ( and only monster melee attacks - player melee attacks are just regular ol' hitscans with tiny range ) depend entirely on line of sight. So, if you set a line to block line of sight ( which can be done on its own in UDMF, but is also included in "block everything" if you're mapping for an ancient version of Skulltag for whatever reason ), then not only will the player not be melee'd while that kind of line is between them and a monster, but the attack will also "miss" if it was already started but didn't get to the attack codepointer until after the player already moved behind the line.

Thanks!