Wasn't this a Ravenloft game? The same Ravenloft that in prior editions had setting rules that weakened Turn Undead and many divine spells for Clerics and Paladins, and then hit Paladins with the nerf hammer of doom by weakening their Detect Evil, disease immunity, fear immunity, had their special mount automatically become evil, and made them detectable by darklords from a mile away? If you wanted to play a paladin it would be borked, and you might want to consider playing a fighter interested in religion instead.
I can understand reservations about having a character who draws power from dark source in a setting where the world is made of evil and fear. Opening yourself to that connection would be like trying to take a small sip of water by standing under Niagara Falls. Every use of their powers would have a chance of moving the character along the path of corruption, resulting in a character that either have to be incredibly conservative about using their powers at all, or would rapidly be corrupted to the point of being an NPC.
There are times when a character concept just doesn't fit the campaign setting. A half-dragon with a fire breath is going to have a hard time in a Spelljammer game based on spending most of your time in the phlogiston, and an aquatic elf is going to have a hard time in an Al Qadim game set in the desert, but neither would be flat out prohibited due to technology or cultural restrictions.
Funny you should mention that.
I was working on my own Ravenloft Placeholder rules, which took into account the restrictions of editions past. In doing so, I noticed a trend in Ravenloft rules: as the editions got higher, they somehow felt obligated to pile on more-and-more restrictions to each class when they reprinted the rules. If one goes back to the original box sets (either the black or red), there were few direct nerfs and most were situational (IE: most immunities fail vs a dark lord or domain ability). Paladin's lost disease immunity, clerics and paladin's turned at -2 levels, bards halved their "know stuff" percentile, and animal companions couldn't challenge a lord, but that was it. Spells were nerfed (or in some evil cases, buffed) but you could avoid that by not casting those spells. Both the priest and wizard spell list were full of non-affected spells. Even then, most fell into a few hard rules (no easy escapes from RL, no plot-ruining divinations, no summons, and weakened anti-undead spells).
Thing is, come about Domains of Dread, the tone changed. The idea was that PCs needed more "gimping" to make the game feel deadlier. Whereas before only a few classes got some abilities restricted, ALL classes got whacked with the nerfstick. In some cases, a simple swap was made (fighters lost their followers, but gained a weak ability to inspire others), or the classic nerf was strengthened (clerics and turn undead) and in some cases (bard, druid, paladin) the class was simply banned for natives. Unfortunately, Arhaus dialed this up to 11 with the d20 Ravenloft: classes got restrictions so heinous as to render them unplayable (barbarians making power checks whenever they rage: what's the point of being one?) and the game took less of the "stranger in a strange land" vibe of earlier boxes and reduced it to a mechanical bull-ride: last as long as you can.
That said, even if the DM here was a fan of the 3.5 Ravenloft Player's Handbook (also known as the "Make a Power Check every level" edition) gimping a bunch of classes with the short-rest restriction is just... inelegant. There is no thematic reason, and Ravenloft never restricted rest before (well, one could draw that out of the psionic PSP rules, I guess). Sounds more like a "I don't like this new rule, therefore I'm going to do all I can to make it unusable".
FWIW, "2 short rests per day, with at least 8 hours between them" is redundant, assuming you take at least one long rest each 24 hours. Your long rest is 8 hours, leaving 16 other hours in the day. If there has to be at least 8 hours between short rests, that only leaves 2 possible before the next long rest comes up.
The math still doesn't add up to two serviceable rests. RL days are 24 hours.
0:00 (Midnight) to 8:00: Long rest (8 hrs)
8:01 to 10:00: three hours of adventure
10:01 to 12:00 short rest
12:01 to 19:00 eight hours of adventure (no rests)
19:01 to 20:00 short rest
20:01 to 23:59 three hours of adventure
0:00 new day, time for bed.
This assumes a short rest two hours after the long rest and two hours before; if you use your short rest five hours after your long rest, you cannot short rest before you can use your long rest. Therefore, while you could (in theory) fit in two short rests if you use your first within the first four hours of the day, and only if you want your second rest before bed.
Perhaps the DM doesn't want to make resting as difficult as what the DMG may offer as alternate resting rules for grittier games (short rest is 8 hours once per day, long rest is weeklong rest in town as has been discussed in other threads.)
Ironically, that would make his character STRONGER than the wizard or cleric; 1 spell slot refreshed daily vs. 2 for a whole week (or more)?
And lets not kid ourselves here. Any one of us could run a Ravenloft game with 0 rules changes and still run it 'Gritty' and 'dangerous' and 'gothic'. House rules like this are a lazy/ignorant DMs tool.
Ravenloft still requires some house-ruling, but this is a ham-handed way to keeping PCs meek.
