AJ Towns posted this on the Bitcoin Developers Mailing List today:

Hello world,

I propose mining testnet 5's genesis block at the end of the week, aligned with the next mainnet retarget at block 975,744.

If people want to point hashrate at bootstrapping testnet5, I've got some docs on the process available at:

  https://gist.github.com/ajtowns/ebb148819c6028387271e6f0dce5e1da

and the 0%-fee solo-mining pool to point to is available at:

  https://lapis.erisian.com.au/chain.html

It will keep running tests at lower difficulty for the rest of the week until shortly before block 975,744 is mined.

Testnet5 will launch at a higher difficulty than Testnet4 did, and (I believe) it has the same proof of work rules as mainnet (unlike Testnet3 which had an infamous difficulty reset). I believe BIP 54 will be active from block 1 on Testnet5.

The genesis block can commit to the mainnet header at height 975,744 (expected about 2026-11-13 18:00:00 UTC) or later.

If you are interested in pointing some hash power at Testnet5,

lapis.erisian.com.au is running a ckpool instance for mining the genesis block for testnet 5

But the pool will not give out work until the above-named blockheight.