In MAMP6 (which used Apache 2.4.54), configuring server side includes could be done by modifying the httpd.conf file and adding
AddOutputFilter INCLUDES .html
However, in MAMP7 (Apache 2.4.58), this does not seem to work.
Checking Apache Server Information (Tools > Apache > Info in MAMP7), has the same info as in MAMP6:
In file: /Library/Application Support/appsolute/MAMP PRO/conf/httpd.conf
626: AddType application/x-httpd-php .php .phtml
632: TypesConfig /Applications/MAMP/conf/apache/mime.types
...
675: AddType text/html .shtml
676: AddOutputFilter INCLUDES .shtml
677: AddOutputFilter INCLUDES .html
Thanks to this StackOverflow answer:
Apache Error Log - an unknown filter was not added: includes
It led me to the Settings dialog, Server tab, where I had to check "include_module" and restart Apache. Once that was done, SSI worked!