Installing PostGIS on older homebrew versions of PostgreSQL (e.g. postgresql@9.6)?

As there are two ways to solve this question, I asked for another approach here.

Whenever a major release of PostgreSQL appears, homebrew will happily install it right away, unless one pins a certain version or installs a specific version from Homebrew/core.

So far so good, but the problem is that PostGIS also keeps updating and eventually will be incompatible with pinned or Homebrew/core versions of PostgreSQL. Again, one can pin PostGIS, yet Homebrew/core does not offer previous versions. From time to time I do brew cleanup and somehow I am now left with a broken PostGIS database. Ok, so "time to jump to PostgreSQL 10.1", I though, but pg_upgrade now gets stuck with the error

ERROR: could not access file "$libdir/postgis-2.3": No such file or directory

That means I can't back nor forth: I cannot figure out how to get PostGIS working with postgresql@9.6 and I cannot pg_upgrade to the new version of PostgreSQL (10.1).

Therefore my question can be answered in two ways. Here I am asking:

How to install PostGIS with older PostgreSQL versions, i.e. postgresql@9.6, using homebrew?

The second approach is described at Upgrading PostgreSQL with broken PostGIS installation ("$libdir/postgis-2.3" error)?

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Tried so far:

• This gist offers a solution and it might actually work. However, I had to use ln -s instead of cp -a. It seems to be working now, but unfortunately I have the problem that my database is already on 2.4 and there seems to be no downgrade path. Will keep trying.