This is pure text region in latest versions.

You can enter a subscript to your text while in it.
You need to enter just that portion with Insert > Formula.
It will depicted as a formula but attached inside text region.
For example; this \[REGION[eb2859b9fe594c78ba46123b695bdc00]]\is an example.
Even better, text region can do your calculation inline.
If I type \[REGION[25bdb2c2da8d44a183f60019b778de61]]\or \[REGION[17864d4a559c45b1bc1a6bfa35ab0717]]\ and don't evaluate it won't calculate it.
It will do calculation if I do click = or type = after expression.
This \[REGION[46aeb8b2b62f47a7bfb93fb2f84292f3]]\ and \[REGION[5a9b28e3c533422488a74476ba32acd3]]\ is an example to that use scenario.