Two Steps Back

Modern Minecraft has definitely cemented itself as an experience distinct from day of old. After taking notice of some mods intending to restore that retro feel, I have compiled a list of mods and resource packs which truly send you back in time. 

Modern Beta

https://www.curseforge.com/minecraft/mc-mods/modern-beta 

Allows for classic world generation. I recommend Beta generation with modern biomes.

Golden Days & Simpler Times
https://www.curseforge.com/minecraft/texture-packs/golden-days   ~   https://resourcepack.net/simpler-times-resource-pack/
Nearly completely replace modern textures with more nostalgic ones.

Nostalgic Tweaks

https://www.curseforge.com/minecraft/mc-mods/nostalgic-tweaks
Brings back a plethora of old mechanics.

These additions (among others) restore the feel of Beta Minecraft, but a bit more was needed to round off the experience.

Amethyst Equipment, Copper Armor, & Emerald Extended
https://www.curseforge.com/minecraft/mc-mods/amethyst-equipment   ~   https://www.curseforge.com/minecraft/mc-mods/copper-armor-tools-fabric   ~   https://www.curseforge.com/minecraft/mc-mods/emeraldextended

Bring about much more variation in gameplay.

Bamboo Combat
https://www.curseforge.com/minecraft/mc-mods/bamboo-combat

Isn't entirely apparent, but use of their spears brings a unique and classic feeling new weapon.

Magic Combat Wands
https://www.curseforge.com/minecraft/mc-mods/magic-combat-wands

Expanding on classic combat was a big goal of mine, and this new class of weaponry helped quite a bit.

All five of these mods are also retextured to fit the theme. An exclusive resource pack makes sure of this.

Many more mods complete the experience. A plethora of optimization mods keeps the gameplay as smooth as possible. Several unstated gameplay additions give the experience even more depth.
Now go on, and see for yourself how Minecraft could have turned out.