This past week, USCIS announced that they will no longer approve I-526 petitions on a first-in-first-out basis but rather according to the visa availability approach. Could you please elaborate on this very important change?

Absolutely. According to USCIS, this new operational approach aligns with other visa-availability agency adjudications processes. It is more consistent with congressional intent for the EB-5 Immigrant Investor Program. It also increases fairness in the administration of the program.
USCIS Deputy Director, Mark Koumans, said that changing our approach from a first-in, first-out adjudication process to one that prioritizes petitions connected to individuals from countries where visas are currently available better aligns the EB-5 program with congressional intent, and makes it more consistent with other USCIS operations. He also added that this new approach increases fairness, allowing qualified EB-5 petitioners from traditionally underrepresented countries to have their petitions approved in a more timely fashion to receive consideration for a visa.
This operational change is consistent with the agency’s processing of Form I-130, Petition for Alien Relative, in cap-subject categories. The new visa availability approach simply gives priority to petitions where visas are immediately available, or soon available, and will not create legally binding rights or change substantive requirements. Applicants from countries where visas are immediately available will now be better able to use their annual per-country allocation of EB-5 visas. The new visa availability approach will apply to petitions pending as of the effective date of the change. USCIS will implement the visa availability approach on March 31, 2020.
Countries such as mainland China might be negatively impacted by this change. Previously, even though applicants had no chance for adjustment of status or scheduling a consular interview, they were having their I-526 petitions approved, simply due to the relatively early date of their I-526 filing. As a result, applicants from countries where there is visa availability were having to wait unnecessarily long periods. For example, as of today, USCIS is posting 32.5 to 49.5 months for current I-526 processing times. A couple of years ago, this number was significantly lower. Hopefully, we expect this change, when implemented, to bring processing times for applicants where there is no backlog to be more in line with the former waiting times, as opposed to what we are experiencing now.