There
was an altcoin back in 2018 called Bitcoin Flash (BTF)... but it's a dead project as far as I can tell.
So, I've no idea what this particular incarnation of "Bitcoin Flash" is.
From what you've described, it seems like some sort of mechanism for scamming newbies who don't understand the dangers of zero confirmation transactions... whereby you can convince them that you've sent them X BTC (perhaps they have some sort of fake "blockexplorer" etc)... You then get them to send a bunch of goods, altcoins or fiat etc... and then the transactions all "disappear" after 90 days, by which time you've also disappeared.
Hopefully this thread gets index by Google so when people start googling "Bitcoin Flash" they find this thread.
I ask just for this, could a transaction or a chain of transactions stay in the mempool for 90 days ? the longest I've seen was 21 days.
Theoretically, you could push a transaction (or chain of transactions) into the mempool indefinitely... and when I say "the mempool", you have to remember that there isn't just "one" giant shared mempool, each node maintains their own mempool (with their own acceptance/rejection and retention rules).
But, assuming your transaction does not get included in a block or becomes invalidated by UTXOs being spent/confirmed in other transactions... someone could just keep rebroadcasting the transaction indefinitely... and your transaction could just keep floating about forever.
If the mempool emptied out and/or fee rates were back to 1sats/byte levels... this would in extraordinarily difficult to do tho.