Padding files are not restricted to hybrid/v2 torrents:
‣ v1-only torrents can have padding metadata, as per the bittorrent's implementation back in 2016 with BEP 47.
‣ V2 and Hybrid torrents were introduced using metadata for v1-compatible clients and merkle tree for v2-compatible clients with BEP 52.
‣ Most clients without v2 support still offer the option to optimize piece alignment through BEP-47 compliant padding files.

How BEP-47 compliant torrents are handled by popular clients:

PROBLEMATIC:

TRANSMISSION (Tested Buid: 4.1.1)
● Compatible with Hybrid torrents: No
● Compatible with V2 torrents: No
● Torrent Creation: v1 (no padding metadata)

‣ Doesn't handle padding files.
‣ Padding files show up in the file list.
‣ They can be selected for download.
‣ If padding files exist in a seeder's PC they will be transferred.
‣ If no seeder has the padding files the download will never finish.

Verdict: Impossible to finish downloading. Impossible to seed even if all content exists on disk.

WORKING REGULARLY:

UTORRENT (Tested Build: 3.5.5)
● Compatible with Hybrid torrents: Yes (Fallback to v1)
● Compatible with V2 torrents: No
● Torrent Creation: v1 (no padding metadata)

‣ Padding files show up in the file list.
‣ Padding files are unchecked by default, but can be selected.
‣ When unchecked, a partfile.dat will be created to handle the padding metadata.
‣ When checked, padding files will be created in the download path.

Verdict: Only torrent content is transferred/required in order to seed.

QBITTORRENT (Tested Build: 5.1.4 with Libtorrent 1.2.20)
● Compatible with Hybrid torrents: Yes (Fallback to v1)
● Compatible with V2 torrents: No
● Torrent Creation: v1 (padding metadata is optional)

‣ Padding files will not appear in the file list.
‣ No padding file transferred or created on disk.

Verdict: Only torrent content is transferred/required in order to seed.

QBITTORRENT (Tested Build: 5.2.0beta1 with Libtorrent 2.0.11)
● Compatible with Hybrid torrents: Yes (v1/v2 choice is peer-dependent)
● Compatible with V2 torrents: Yes
● Torrent Creation: v1/hybrid/v2 (v1 without padding/v1 with padding/hybrid/v2-only)

‣ Padding files will not appear in the file list.
‣ No padding file transferred or created on disk.

Verdict: Only torrent content is transferred/required in order to seed.

BITCOMET (Tested Build: 2.20)
● Compatible with Hybrid torrents: Yes (v1/v2 choice is peer-dependent)
● Compatible with V2 torrents: Yes
● Torrent Creation: Hybrid (Padding Metadata + Merkle Tree)

‣ Padding files will not appear in the file list.
‣ No padding file transferred or created on disk.

Verdict: Only torrent content is transferred/required in order to seed.

*Tests were conducted with v1-only and hybrid torrents created by me using qbittorrent and bitcomet.