I have a very light modpack I set up for personal use that uses the mods "Modern Beta" and "Nostalgia Tweaks" to recreate the atmosphere of old school MC (and also bring back and remove a lot of features) while still having a lot of the things in new MC that I do like (sprinting, blue dye from flowers, no more terrifying sleep glitch when a skeleton appears in my house and shoots me awake, cats, villages, better wolf AI, mordern redstone mechanics like hoppers, corner stairs, chests side by side...).

Modern Beta lets you generate worlds using terrain generation from Early Alpha, Beta 1.7.3, and even several version of InfDev, Classic, and OG xBox terrain generation, while also allowing you to fully customize them to have (or not have) modern biomes, structures, features (like andesite and its ilk) and it even has "beta-fied" versions of new biomes if you still want them in your world but dont want them to harsh the aesthetic of your world.

Nostalgic tweaks is awesome because it has SO many options. You can disable hunger while still having sprint, you can even straight up disable sprint if you want. You can remove experience, or change experience to work like it did when it was very first introduced. You can completely nix enchanting tables and/or anvils from the game. You can bring back the boat/water elevators from alpha, the ability to place ladders with a gap in between, cart boosting, even that good ol' milking squids glitch! It has tons of options for visuals, bringing back old light rendering, the old menu screen, removing things like realms and such options from the menu, you can even bring back the version overlay in the corner of the screen, and customize what version it shows. You can stop furnaces from even having a sound when cooking, or change the sound doors make when placing them, or even removing the sound of chests opening. You can bring back the old chest models that don't even visually open, or change the hitboxes of fences back to when they took up the whole block space.

That's how in depth you can go with customization. You can even customize how things stack, so if you remove hunger and want to bring back non-stacking food items so you cant just cary 64 steaks to endlessly heal yourself, and you actually have to be smart about food carrying and storage, you can! It's also still being worked on, so any features missing will likely be added still.

I love playing older MC versions but I have been spoiled by modern features, as much as I hate to admit it, and while there are ways to mod some features back into old MC, it's not perfect. I haven't played MC as much as I have since I discovered those two mods. Also the Golden Days resource pack which not just brings back old textures and sounds, it also retro-fies new textures so they dont stick out. It's great because it really just feels like playing some kind of vanilla+ modpack from the beta days, rather than playing a retro-fied version of modern MC.