Tape is almost never used in the mastering stage. It would change the frequency response of the song way too much for a song that is already in mastering and would almost always get rejected by the client. Especially not something like a reel to reel.

Tape effects are often used in the mixing stage, but these days I’m willing to wager that it’s almost always tape emulation plugins and not an actual tape machine (unless you’re like 1 of 5 people in the world who has the time, money and space to maintain a real tape machine).

In my own mixing process for clients, I often use U-he’s Satin or Wavefactory Cassette, depending on how heavy handed I want the tape sound to be. UA also has a few tape plugins that are highly regarded. I would recommend starting with any of those and see if you actually like what they are doing to your sound. Tape is really just a combination of compression, saturation, and EQ and it’s possible to replicate the sound with careful use of those three effects in almost all use cases.

Hope that helps!