For someone already in USA on H1B, I expect there won't be much impact other than added delay, as the processing of H1B within USA is done over mail and not involve interviews.
Why would current H1Bs be impacted? They already have their non-immigrant visa no?
They'd be impacted if their visa is due for renewal.
Britain has been extended some visas, recognizing that it's sometimes unreasonable (person from a country with more cases) or impractical (lack of flights) for people to leave when they expected to.
The current extensions are only to 31 March, but maybe they will increase again after that.