Magic items, especially weapons, were mostly given out like candy in AD&D (check any official published adventure from that time). Sounds like you had an awesome experience, but lack of access to a magical weapon would not have been a plot point in most campaigns, and there is nothing stopping a current DM creating a low level monster that can only be hit by magic items.
The big criticism of the published adventures was their absolute failure to follow the treasure guidelines.
I thought immunity to non-magic weapons was basically a joke category even in 5e 2014, which has lower magic item reliance, generally speaking, than AD&D did (AD&D relied on magic items for much of the character progression that is now worked into class design). It was extremely rare with low level foes, and was basically a ribbon ability for most of the monsters who had it.
Didn't you stop and think about the situation? If something seems senseless, meaningless or wasted - it is often a great idea to stop and ask yourself if you might be missing something. Don't just brush it off as someone else being wrong/bad/dumb ... look for meaning and ask yourself if there is something you might be missing.
If you followed the guidelines for treasure provision in the 2014 DMG,
most parties did not get a magic weapon until around level 5 or 6. Some got it As early as level 1. Some didn't get them until you were pretty deep into the second tier. But, using the tables and following probability ...
12 out of every 100 hordes rolled on table F. 23 out of 100 numbers on table F were magic weapons. You got 1d4 items.
3 rolled on G for their hordes. 24 out of 100 on table G were weapons. Only one item.
Let's combine those and be generous and say that 15% of hordes had a 25% chance to having a magic weapon. You got 7 hordes. If you pulled items in the horde you got 1d4 items. 0.0375% is the chance of pulling an F or G horde and then finding a weapon in it as one of your weapons. That comes out to less than a 50% chance by the time you reach level 5. That gave you four levels where it was unlikely you'd have a magic weapon.
Jackalweres, with their CR 1/2, were meant to be used during these levels when magic items were infrequent if not non-existant.
If you want to take it further, between levels 5 and 10 you get 18 hordes. 14% have 1d4 Fs, 4% have 1d4 Gs, and 2% had 1 H with a 19% chance to be a weapon. The odds are not that much greater that you'll pull one here, but cumulatively you pass the 50% mark likelihood of pulling one sometime in level 5 or 6 if hordes are distributed roughly proportionately across levels (depending upon whether you distribute them just by level, or by the amount of time expected to be adventuring at each level as calculated by reverse engineering encounter design recommendations, monster experience, CR, EL, etc...)
Yes, the publishers didn't follow their guidelines. Yes, a lot of DMs just used published adventures and missed out on how the game was designed to work. HOWEVER, the creators are on record discussing the intent of these abilities and how they should be used ... which as I discussed above, is something you should have picked up on. Why have an ability if it is meaningless? You put a mechanic in a game to give it meaning. If it has no meaning, consider that you might be missing something.
If you go back on Enworld you can probably find 200 posts over thew last decade where I go into these phenomena and suggest DMs follow the guidelines. At least once a month during my active periods ...
When I worked with a new DM, it was always something I worked into conversation to let them know how published adventures and the guidelines differed. Some DMs considered my words and what was in the books - and used the guidance. Others gave me a +3 weapon at level 1 because that was fun to them. I did have fun in both types of games ... but the games worked better and were more dynamic far more often in the games that followed the guidelines.
A lot of people complained 2014 didn't work well but NEVER played per the guidelines in the book. If you followed all of them, including how to make easy encounters interesting and using the treasure as sparingly as indicated ... the game was much better. I felt like a lot the complaints I saw in 2014 were from people complaining that their Soda Stream made lousy coffee.