This is an extremely potent Nerf to Rogues
As someone who is currently playing an Assassin-subclass Rogue, that class is balanced around dealing moderate amounts of damage normally, not having the same utility as Thief-subclass Rogues in that they cannot ignore the restrictions on Magical Items like a Thief can, but, in-exchange, dealing huge first-turn NOVA damage. A Level-17 Assassin can use Death Strike:
Starting at the 17th level, you become a master of instant death. When you attack and hit a creature that is surprised, it must make a Constitution saving throw (DC 8 + your Dexterity modifier + your proficiency bonus.) On a failed save, double the damage of your attack against the creature.
This is designed to stack with the Assassinate Feature:
Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have Advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit.
So that an Assassin Rogue could deal what is essentially 4 times damage on a Surprised Target. That is supposed-to-be their ultimate class feature.
By-comparison, a Thief-subclass Rogue gets to take 2 Turns on the first round of combat with Thieves' Reflexes:
When you reach 17th level, you have become adept at laying ambushes and quickly escaping danger. You can take two turns during the first round of any combat. You take your first turn at your normal initiative and your second turn at your initiative minus 10. During each of those separate turns, you get an action and a bonus action, totaling two of each on your first round of combat. Because these are considered two separate turns, you may also Sneak Attack per the normal rules (if eligible) on each turn. That said, you can’t use this feature when you are surprised.
Both are designed around the inclusion of Sneak Attack Dice in the dice doubled for a critical hit. However, if you remove Sneak Attack Dice from the effects of the Critical, one of these is much more impacted than the other:
The Assassin Rogue essentially deals 4x weapon damage and only 2x Sneak-Attack Damage, and, if your DM is particularly mean-spirited, then they might not even allow the 2x Sneak Attack Damage from a failed Saving Throw for Death Strike. What goes from a spectacular First-Turn feature for the Assassin turns into a mediocre feature that can only be used on the first turn of combat. The Thief Rogue, however, still gets to perform two full turns on the first round of combat, so it's not nearly as much of a nerf to them.
And the Thief-subclass can still use any Magical Item regardless of Race, Class, or Level Restrictions with their Use Magical Device feature:
Use Magic Device
By 13th level, you have learned enough about the workings of magic that you can improvise the use of items even when they are not intended for you. You ignore all class, race, and level requirements on the use of Magic Items.
And, if you compare this to fighters, who get spectacular damage regeneration features (Champion), multiple-attack features (Eldritch Knight), and going so far as to Ignore falling unconscious and interrupt the current turn when dropping to 0 HP without dying outright (Xanathar's Samurai).
In the end, combined with the effects in This Question, it strongly seems like this DM is trying to railroad your party into playing Fighters, Barbarians, and Monks.