Will there be an Update of 1.5.0p23?

do we have to upgrade to 1.6 or will there be updates of the 1.5 in future ?

Thank you

Yes. You can see the end of life here:

Version Release-Date End of active maintenance End of passive maintenance
1.6.0 2019-09-24 2021-03-17 2022-09-08
1.5.0 2018-08-01 2020-02-01 2021-08-01

And the fixes, which will be in the patch release here: https://checkmk.com/check_mk-werks.php?edition_id=enterprise&branch=1.5.0&version=1.7.0i1&werk_search=

Keep in mind, these are only patch releases with bugfixes.