I expect this won't apply to the OP, but one airport to be aware of that currently isn't included in the TfL scheme is Stansted, to where Greater Anglia operate services as Stansted Express from Liverpool Street. You can tap in at Liverpool Street, but beyond Tottenham Hale (the first stop) Oyster/contactless payments are not valid so you cannot tap out.

There are warning signs at various stations en-route, but clearly many people miss them because there are plenty of reports of passengers being charged penalty fares.

There has been a proposal to extend the network to include Stansted for some time, but it seems no nearer realisation.