Legal is such a crappy term when applying it to game TOS/EULA. Allowed would be more apt.
From Lavishsoft's perspective - it is very legal to use ISBoxer, provided you are paying for an Inner Space subscription.
From a game publishers perspective - it is "legal" if it does not break the TOS/EULA.
Almost every TOS/EULA can get a bit fuzzy around the:
"change how you play", which effectively bans any new method of computer interaction that was not specifically designed in the game. Stinkyboards, touchscreens, and VR headsets are running that thin line.
"hacks, cheats, automation software, unauthorised third-party software", pretty fuzzy here. No-one ever publishes a list of authorised third party software, so this effectively bans Windows, drivers, applications, and everything else necessary to actually play the game.
"read/modify game files and/or game process memory" - easy one here, ISBoxer doesn't do that.
...and they go on....
If you want an actual "legal" opinion, then just about every game is unable to be played at all without breaking the TOS/EULA.
Essentially it comes down to whether a specific publisher tolerates multiboxing, and a few of them do (Blizzard being one of them for specific games), a few of them don't, and some are wishy washy and change their stance and/or limitations over time (not looking at anyone in particular CCP). Normally the question is placed on the appropriate game forums to find out, or if not expressly limited in the TOS (usually to 1 account per person), and there is no answer to a forum post from someone with actual authority, someone will multibox it and find out.
Ones we know that it is allowed (or not), potentially with limitations, they are usually listed on the
Supported Games list.