The earlier stay to 25 June 2020 brought in when lockdown began has been extended to 23 August 2020.
The new Rule 55.29 has been brought in and stays all possession proceedings which are already stayed; so those which have been automatically stayed are caught; and also new possession proceedings issued on or before 22 August 2020.
The case of Arkin v Marshall  EWCA Civ 620 considered the purpose of the stay and was quite firm in that the stay was lawful and lawfully implemented.
Most importantly the Court of Appeal held that a court could lift the stay but:
a judge retains the power to lift the stay which it imposes. But the proper exercise of that power is informed by the nature of the stay and the purposes for which it was evidently imposed. PD 51Z imposes a general stay on proceedings of the kind to which it applies, initially subject to no qualification at all, and subsequently qualified only in the limited and specific respects provided for in paragraph 2A. The purpose was that during the 90-day period the burden on judges and staff in the County Court of having to deal with possession proceedings, which are an immense part of its workload, would be lifted, and also that the risk to public health of proceeding with evictions would be avoided. […] Thus, while we would not go so far as to say that there could be no circumstances in which it would be proper for a judge to order that the stay imposed by PD 51Z should be lifted in a particular case, we have great difficulty in envisaging such a case.(at p.42)
Therefore it seems that this stay, made by amendment to the rules is on a slightly different basis (the pilot process in Part 51 having not been used) but the same considerations are likely to apply.