No things are "missable" concerning vanilla settings. It's pretty much how it used to be. One difference is that the materias in the final cave are moved to the materia caves.
The real "missable" things are my new random enemies. There should be one new enemy in most places.
Exceptions are, that is, areas without new enemies (from the top of my head):
5th reactor.
Train graveyard.
Mythril mines.
Wutai.
Temple of the ancients.
Ancient city/sleeping forest.
Glacier (the snowy maze-like area before gea's cliff. Not including gea's cliff)*.
Underwater reactor**
Rocket town
*In gea's cliff there is one new enemy as you climb. A new one where you encouter giants and black bombs. And finally a new one in the crater (red pyramid).
**Not in the reactor, but before the elevator to the underwater area there is one along with the sweepers.
New enemies on world map.
Beach outside midgar/kalm. Beach junon. Beach gold soucer or forest path costa del sol. grass area outside gongaga, nigelheim, rocket town and mideel. beach temple ota. outside icicle inn. And outside purple materia cave.
Sorry if I was a little vague
What exactly I wanted to know was if the missables were similar to vanilla.
Ex: Enemy skills such as Trine, weapons and armor like Missing Score and Escort Guard, and Materias such as summons.
Things like that