Arial, verdana, etc, all have capital "I's" that look like l's. Is there a font that is plain, with small to no serifs, that has capital i's that look like - I?

Those arms on a capital "I" are... serifs. That's why you're running into issues. For the most part, and for most major fonts, you'll only see those arms on serifed fonts.

As in...Times New Roman, for example.

There are innumerable sans serif fonts where uc /I has arms.

For one:
Harcourt Education/HeinemannSpecial-Roman [[http://www.myfonts.com/fonts/fw-heinemann/heinemann-special/]]

Here are some more names -- other than the armed /I and being sans serif I know nothing about any of them:

Krebs, Manuel/Replica-Mono
Kreative Software/Miranda 25
A Lee/Ongdalsam
Global Village/FocalPoint On Line
Excalibur Monospace

Here's a good list of sans-serif fonts with capital i that has horizontal arms (crossbars): [[https://andarnoespirito.wordpress.com/2022/02/09/sans-fonts-that-show-uppercase-i-with-crossbars-to-differentiate-from-lowercase-l/]]

I have a plain font called Semi-Casual that has horizontal arms on the capital I: https://www.dafont.com/semi-casual.font

Have a look at Transport Medium and related fonts. Designed for use on road signs, it is unusual in having capital I, lower case l and digit 1 that can’t be confused with each other.