What is it called when near the end of the song there's a section with a deliberately slow tempo?

Question

No, not a final gradual ritardando. It's hard for me to describe. It may only be a thing in musical theater. Here are some examples:

• 2011 Tony Awards Opening Number (starts around 4:45)
• "Gee, Officer Krupke" (starts around 3:45)

Answer 1

This musical section is referred to colloquially as the "kick line." (see also The Rockettes)

It is characterized by 4/4 (or equivalent) meter, and a tempo drastically reduced from whatever came immediately prior, often accelerating gradually to eventually match that of what came before.

You're correct that it is specific to musical theatre; the name comes from the choreography that often accompanied it of a line of dancers doing unison kicks in time with the music.

It shows up at the end of songs because this choreography would often be used for a "big finish".

This name applies equally to the musical idiom as well as the dance idiom, even if the choreography is totally different (in the case of the Tony Awards clip) or not present at all (in the case of the West Side Story clip).