Yes, we removed it back in 7.0.175 or thereabouts to address rounding / reconciliation issues and
arrived at the conclusion that you don't make FX payments by entering in FX rates to work out the local currency amount.
In fact, we made the field read-only for a few builds until we
opened it back up for edit again.
You need to enter both the FX Amount and the local Amount and we impute the rate. We do let you enter the rate, but doing so just calculates the local amount from the FX Amount - but the only way to avoid reconciliation issues is to enter the 2 amounts instead of the rate.
When you're paying an FX invoice via that form, you know the amount in FX you are paying because that's what you told your bank / payment facility to pay in FX currency - and they'll tell you what that worked out to be in local currency - in fact you should use whatever appears on your bank statement / transaction record. The rate is imputed and we use that to work out any FX gains or losses.
Make sense?
If it's important for the customer, it should be a super easy plugin to add back that function.
EDIT: I found the original issue calling for the removal of the rate -
DEV-5582 which has a comment linking to a docs article which apparently explains things further.