The points at which the game transition between acts seem a bit arbitrary (mainly for Act I to Act II), and I don’t see a narrative or mechanical reason to lock us out of previous maps and quests. As far as I remember, previous Baldur’s Gate games didn’t have this kind of points of no return. Why do you think they did it? Do you like it?
There isn’t a point of no return between A1 and A2. Rather, you need to do certain A1 events prior to A2 or those events resolve without you (e.g. Goblin Attack, several others).
You can travel back to A1 while in A2. Only A3 prevents back-travel, and that’s for well-established story reasons.
I generally prefer having control of the timing rather than having it being tied to X amount of rests or something (notable exceptions for certain quests existing, of course).
BG2 and ToB both had notable points of no return. Paying the fee in Athkatla to recruit allies is one, any time you ever travel with Saemon is one, leaving the besieged city is one in TOB, etc. There were also a number of time-dependent quests, most notably those involving a certain dragon/drow situation.
You do get locked out of traveling back to Act 1 areas some time in Act 2. I think once you enter the Temple of Shar?
I wanted to go back for another crack at the forge and I couldn’t.
It’s when you go to Nightsong that it locks you out, and to be fair, the game does clearly warn you there are consequences for that - and depending on your actions, a 3rd party just straight up moves the story along, so it makes sense.
If you’re struggling at the forge, remember that you can just not hit the boss with anyone but a bait character, and that bait character doesn’t have to actually survive the encounter, meaning if you position well, you can just melt and smash the boss over and over without touching it (after the first hit or two) and it’s a free win.
Took me numerous tries before I learned that one.
The actual cheese of the forge is picking the pike of returning, placing one character in the stairs, waay above, and throwing the pike into him. If it’s a fighter that’s 4 throws the first turn, each doing their respective damage + 30ish damage per throw because of the weight+height. I killed it in 2 turns, he didn’t even move since he does nothing in the first turn.