There is only one LTS per year, so the question is mostly about release timing, when OpenWrt has been able to migrate enough targets/devices to the current/newest LTS.
(release 18.06 was made with split kernels 4.9 / 4.14, as the migration had progressed slowly for some targets.)
E.g. right now the migration to 6.1 is (slowly) progressing, so it makes not much sense to talk about further kernels like 6.4
Longterm release kernels / OpenWrt release comparison
| Version | Released | Projected EOL | OpenWrt version |
|---|---|---|---|
| 6.1 | 2022-12-11 | Dec, 2026 | OpenWrt 24.x ? |
| 5.15 | 2021-10-31 | Oct, 2026 | OpenWrt 23.05 |
| 5.10 | 2020-12-13 | Dec, 2026 | OpenWrt 22.03 |
| 5.4 | 2019-11-24 | Dec, 2025 | OpenWrt 21.02 |
| 4.19 | 2018-10-22 | Dec, 2024 | - |
| 4.14 | 2017-11-12 | Jan, 2024 | OpenWrt 19.07 |
Practically only 4.19 LTS has been skipped in 2020 when there was no release.