USCIS is not usually involved in determination whether a permanent resident can return to the US. It would be the immigration officers at entry, part of the CBP, or the immigration judges in immigration court, that would deal with eligibility for entry. USCIS does deal with whether a permanent resident has enough continuous residence to qualify for naturalization,
There are two separate issues regarding the ability of a permanent resident staying outside the US for a long time to return to the US: the documentary requirements for re-entering the US, and abandonment of residence.
For the documentary requirements for an entering immigrant, the acceptable documents are listed in 8 CFR 211.1(a). The main ones applicable for a returning permanent resident are: a green card after a temporary absence of 1 year, a Re-entry Permit, or an immigrant visa (which would include an SB-1 returning resident visa). So a permanent resident who has been outside the US for more than 1 year without a Re-entry Permit would not have any document that allows them to return, and, going by the rules, would need to get a returning resident visa to return to the US.
There has been no "relaxation" in these rules, although the existing rules already provide some degree of flexibility. For example, 8 CFR 211.1(b)(3) and 8 CFR 211.4 allows an alien returning to unrelinquished permanent residence after a temporary absence abroad to seek a waiver of the documentary requirement with the officer with jurisdiction over the port of entry, and allows the officer to waive the alien's lack of such a document if they are satisfied that the alien has established good cause for the failure to present such a document. If the officer denies the waiver, and refers the person to removal proceedings in immigration court, the person can renew their application for a waiver before the immigration judge.
As for abandonment of residence, there are no clear regulations on this, and various case law has established that abandonment of residence is a subjective determination that depends on many factors, including the person's purpose for departing, whether the person had a definite date by which they expected to return, the person's place of employment, etc. It does not depend on length of absence alone, and thus there is no need to relax any rules for COVID-19.
For "continuous residence" for naturalization purposes (which is different from maintaining permanent resident status, as someone can interrupt "continuous residence" and still have maintained permanent residence status), an absence of between 6 months and 1 year is presumed to interrupt continuous residence, but this presumption can be overcome with sufficient evidence of having maintained ties to the US. Circumstances surrounding COVID-19 can already be considered in the determination whether this presumption is overcome. However, an absence of more than 1 year definitely interrupts continuous residence (unless one files N-470 which is only for someone working for the US government or certain US organizations abroad), and there has been no relaxation in this 1-year cutoff.