We had this functionality in an early version, however, we made the decision to remove it because it was not possible to handle many different scenarios where there are interdependencies between different schemas, most notably in stored functions/procedures where Postgres doesn't track dependencies because it usually doesn't parse the function bodies itself.
Starting pgadmin 4 (4.25), the schema filter drop down disappeared in the schema compare tool. Now the tool compares the who database, all schemas, no choice. Ouch. What can be the reason for dropping the schema selection in a tool called Schema compare? 4.24 still has the schema filter.