Guessing no I, L, or O to avoid confusion with numbers.

At that point I feel like you should just use base32 instead of 33.

For just removing digits-E-digits, that's 10000 patterns for each E position, of which there are 3. Since 36^5 is over 60 million, you're only removing a twentieth of a percent of values. Closer to a tenth of a percent if you remove those values from 33^5 or 32^5.

It's more useful to look at the total amount of values rather than how much the space gets reduced. If you have 10s of millions of values you can have several devices for even the largest employers that exist today.

It was I, O, and E.