Installing and upgrading help

1) Why isn't it recommended
My reason would be that every production site uses plugins and most plugins are not developed or tested using MSSQL. In 10 years of Moodle development I have only ever once run any Moodle code against MSSQL and that was to debug an issue that occurred because of a slight difference in the way MSSQL works by contrast with the other databases. That is no criticism of MSSQL which is an excellent database system.

There is a whole world of advice on performance and optimisation for the other databases but very little on MSSQL server.

3.  Are there commercial support models for Moodle ? 

Yes, look for the term 'Moodle Partner', organisations that are certified by the creators of Moodle to offer support. Many of the people who post in these forums work for Moodle Partners and have been working for the product for many years. For example I work for Moodle Partner Titus  and You will see adverts on this sites for other partners. You can find out more about the partners here.
https://moodle.com/partners/

The "(recommended)" labels don't make sense to me, I'd ignore them.

To the best of my knowledge SQL Server is fully supported by Moodle by which I mean it should work completely with no functional restrictions. However:

• MySQL and MariaDB is used by the majority of Moodle sites: support is easy to find on these forums and through commercial channels.
• PostgreSQL seems to be used when in-house skills are available particularly with large Moodle sites. Because it's use is less widespread support might be harder to obtain but those organisations are often able to support Moodle themselves.
• SQL Server is probably as widely used as PostgreSQL but where in-house Moodle support skills are more limited, but external support is harder to obtain because of its less widespread use. As an example, there was a problem reported using Moodle on SQL Server (one, two) where the forum couldn't provide an answer, I don't know if either party resolved the issue themselves.
  

When evaluating commercial support options – whether Moodle Partners as Marcus suggests or other providers – try to establish their experience of supporting Moodle specifically on SQL Server.

Marcus' makes a good point about third party plugins, there are some that assume the database will be MySQL/MariaDB so won't work with other databases.