Staple all your tax pages (forms and schedules) together in the upper left corner.

If you are filing on paper by mail, staple the 1099-R to your Federal income tax return ONLY if there is an entry in Box 4 for Federal income tax withheld. Staple it to the front on your return.

For more information on assembling your return, please see the following FAQ:

https://ttlc.intuit.com/replies/4246000

I have been efiling for years, so things may have changed in that time period, but if you look at the 1040 instructions for 2020, there are directions on how to assemble the tax form (what pages go in what order), but there is no mention whatsoever about stapling. I assume that since they didn't tell you one way or another, it is optional.