@jcd4 

1) If you have Quickbooks Desktop 2021, with an indefinite license, after 31 July 2024, will someone be able to open up their company file and use the QBDT 2021 software?

Make sure you have QB Desktop 2021 with a non subscription license, not QB Desktop 2021 Plus.

https://camps.intuit.com

2) After 31 May 2024, transaction ingest through QBO files will be turned off, correct? However, after 31 May 2024, one can still ingest transactions from bank and credit cards through IIF and CSV files, using 3rd party or other available freeware. This is factual, yes?

Yes. If you prefer to keep using the bank feed feature after May 2024, you will need another workaround. Contact @Fiat Lux - ASIA for details.