mariadb – Should a meta_key or other key-value table columns use a HASH or BTREE index?

I’ve noticed that my WordPress database has BTREE indexes everywhere.

By my understanding, BTREE indexes allow more efficient < > and BETWEEN queries, but are otherwise less efficient than a HASH query for = and != queries.

This makes sense for some columns that might be filtered numerically (dates obviously, and IDs potentially, – though in practice ID > x doesn’t seem very likely in WordPress), but I can’t see any benefit for columns like meta_key which stores the text key of a key-value pair, or indeed the post_id column of a post_metadata table, which will almost without exception be JOINed to the posts table.

Would a HASH index be more performant? If so, is there any reason that my database is using BTREE indexes for these columns, or is this just a default? If not, what am I lacking in my understanding of indexes?