Is the following index is good?
Optimal for both queries:
INDEX(roomId, status,   -- first, in either order
      userId,           -- 3rd
      isMuted)

Notes:

In InnoDB, the PRIMARY KEY is tacked onto the end of each secondary index (so it can get to the record).
All 5 columns (including id) are in that index, so it is &quot;covering&quot; for the first query.
In both queries, the tests in the WHERE clause are completely handled by the first few columns.
In the 2nd query, userId is available, so the index is covering for that query, too.  (The extra isMuted` is not harmful.)
I assume that id is the PK of b, so that is optimal, too.
If you add my index, drop combine4.  (The left part exactly matches.)
More:  http://mysql.rjweb.org/doc.php/index_cookbook_mysql