What is the maximal altitude at which an emergency slide can be deployed?
In the US, TSO-C69c details the FAA's technical requirements and testing protocols for evacuation slides. I can't find anything in it that talks about a maximum altitude or minimum air pressure. The closest thing I can see is this requirement in 4.2 to function correctly in a given temperature range:

The device, including its inflation system, must be demonstrated to be
capable of fully functioning when subjected to temperatures from -40
to +160 degrees F. If the device is intended for installation outside
the pressurized cabin, the device must be capable of functioning after
being stowed at -65 degrees F. The function of the device must be
demonstrated in accordance with the hot and cold soak test procedures
described in paragraph
5.9 of this appendix.

Sections 4.11 on inflation operation and 4.15 on the inflation system don't mention anything about ambient atmospheric pressure.
And the testing protocols in section 5 of the document only mention temperature, not pressure. For example, section 5.9.3 is part of the cold soak test protocol:

Deploy the device into ambient temperature conditions (typically
defined as between 65 and 85 degrees F) from the appropriate airplane
door or a suitable airplane door mock-up or module, within 10 minutes
after removal from the conditioning chamber.

4.15.2 has the only reference in the document to an aspirator system and it only requires a filter to avoid ingesting small objects.
So, unless I've missed something in the TSO - which I might have! - maximum altitude/elevation or minimum ambient atmospheric pressure aren't part of the required certification, at least in the US.