Reading through multiple gov websites subject to shutdown, the consensus is shutdown furlough is not treated the same as a sequestration or a position being rendered unneeded. But what else I am seeing is those not excepted from the furlough are prohibited from engaging in any work related actions, including going in their building, teleworking, leaves are canceled, and HR matters are halted. The way I am seeing it, funding for unspecified years can support salaried SES and prepaid contractors still do their thing. Each agency publishes a plan on who stays and who goes, so things have to be looked at agency by agency.
Another fun thing(evil), no second job clauses. Wait until the doors reopen, and be back pay, or move on and never miss a paycheck. Not sure if moving on will get them the back pay as all actions will not be entered until after the shutdown is over. Oh, and the contingency plans are updated all the time.
https://www.whitehouse.gov/omb/information-for-agencies/agency-contingency-plans/