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Support for ZFS in TestDisk is very limited.
TestDisk should be able to find ZFS boot block (struct vdev_boot_header) https://git.cgsecurity.org/cgit/testdisk/tree/src/zfs.c
The beginning of a partition may looks like (hexdump output)

00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ...

At last! I've found it!
1. search for "00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 11 7a 0c b1 7a da 10 02"
2. in data we found, search for "name $ZPOOL_NAME".
3. "11 7a 0c b1 7a da 10 02" should start at 16344 offset (decimal)
4. Offset we need: LOOP_OFFSET = $OFFSET_OF ...

Searching "11 7a 0c b1 7a da 10 02" was not the best idea. After find-counter turned to 8500, I decided to search "00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 11 7a 0c b1 7a da 10 02" instead.
We'll see what will give.

And in his case, ZFS is detected, but noone of my ZFS partitions, both lost and healthy, is visible forTestdisk. Why so?

If your children are kidnapped they are kind of lost but you can never say they are healthy.
The same applies to your ZFS partitions. There is no evidence that they are ...

Disk /dev/sdc - 3000 GB / 2794 GiB - CHS 364801 255 63

Partition Start End Size in sectors
1 P Linux filesys. data 40 4095 4056 [Linux filesystem]
2 P MS Data 4096 2101247 2097152 [T3.D2.E1]
3 P MS Data 2101248 1259382783 1257281536 [Win10]
4 P Linux filesys. data 1259382784 1263609855 ...

Support for ZFS in TestDisk is very limited.
TestDisk should be able to find ZFS boot block (struct vdev_boot_header) https://git.cgsecurity.org/cgit/testdisk/tree/src/zfs.c
The beginning of a partition may looks like (hexdump output)

00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ...

recuperation wrote: 15 Nov 2020, 09:14 Have the ZFS partitions been encrypted?

Neither LUKS nor plain. May be ZFS native encryption, but no third-party soft.

Hello, I've mistakenly destroyed GPT on HDD. (Yes, both main and backup GPT.)
Here: https://www.cgsecurity.org/wiki/TestDisk is said that Testdisk can find lost ZFS partitions.
Testdisk recovered all partition, except 2 non-raid ZFS pools.
It sees even raw partitions, I've backuped in files and raw ...