How I would old school it up
1. No feats
2. No drow, dragonborn, tieflings
3. Variant humans to make them more attractive to increase the human to demihuman ratio, yes it brings in feats but it's one. One feat. Whoopdie doooo.
4. PHB only classes, when you get outside of that, case by case. De-emphasize the magical fighter phenom in post 2e era D&D when you get outside the core rules.
5. Limit multiclassing... or don't use it at all.
6. Use the gritty healing rules
7. Milestone leveling, but every 4-5 sessions or more. SLow that roll. If they are level 10 in a year, you are not old schooling it. If you are doing XP, then reward it for smart play and not slaughter. D&D was a resource management game, not a hack & slash game. Reward XP for getting out of scraps. 2-3 xp per encounter times the CR, minimum 1. Level 1-3 might go quicker than old school but it's a different way of doing it.
8. Scale down the hit points, cap them at level 10 and half the monster HP suggestions when they start hitting about 20-40. Ain't no one got time for 300-500 hit point whittling fights. Cap at about 125.
That's just a few things off the top of my head that I would do to bring it to a more old school level of play. 5E is pretty old school in a lot of ways. It's a logical progression of 2e in the same way 3e was but if different designers handled it. If more people read the DMG instead of watching the less than wonderful youtube reviewers saying "you don't need to read this book" like it's the 2e DMG or something inconsequential, we would see a lot more interesting house rules and stuff I think that could make for some awesome kit bashes.