The Practical Implications of Compatibility

• You’re going to want to pick a core ruleset.
• You’re likely to use the classes and features of that ruleset.
• The concept of using an older or other 5e character class at a newer system (like using a 2014 D&D Fighter at a 2024 D&D game) isn’t very likely.

What stuff is likely to work with minor modifications

• Monsters
• Spells
• Magic Items
• Campaign settings
• Adventures

What stuff is likely to require more significant modifications but can likely work

• Subclasses
• Backgrounds
• Feats
• Races / Ancestries / Cultures

What stuff is almost certainly not compatible

• Playing a 2024 character at a 2014 game. Some rules just won’t work.