I've seen similar posts but none that address how pressure canning changes the flavor. I am trying to make tomato sauce without citric acid because my mom is allergic to it and the foods that contain it because that's how that works (a lot of googling suggest vinegar or lemon juice in place but that's also just citric acid) I saw a similar thread with some debate. I am looking for answers on safety of canning without citric acid specific to tomatos as well as if folks have found the flavor to be significantly altered by pressure canning.I'm new to this so hopefully following the rules well enough!
The purpose of adding acid to canned tomatoes (and tomato sauce) is to lower the acidity of the food in order to inhibit the growth of the botulinum bacteria (a pH of 4.6 or lower does the trick). Citric acid is traditional for this as it doesn't alter the flavor much, but other acids are also usable such as acetic acid (vinegar), ascorbic acid (Vitamin C), or malic acid.
Canning without the use of acid is an option, but it's not a recommended one. That's because food canned without acid is susceptible to botulinum growth. As such, if you do this, you should keep a very close eye on it and it shouldn't be kept for longer than a couple of weeks... which, frankly, defeats the purpose of canning. There's a reason that it's hard to find any tomato canning recipes that don't include acidification of some kind - botulism is extremely serious and should not be taken lightly.
Also, you should have a talk with your mom. Tomatoes naturally contain citric acid, so if she can have tomatoes but not canned tomato sauce, it might be worth investigating what she's actually allergic to.
Pressure canning tomato sauce would likely give a result very similar to a majority of commercially-canned tomato sauce, taste-wise, since pressure-canning is fairly normal at the commercial scale.
However, you'll have to find a process recipe to follow that does not include acidification, which rules out many of the published processes, even for pressure canning sauce. i.e. https://nchfp.uga.edu/how/can_03/tomato_sauce.html clearly specifies non-optional acidification for their processing times.