This post in on the PHP version in v9 only, not on db / browser.
tl;dr: keep php 7.0 support in v9
* php 7.0 is a major version that will be supported by a series of distros even after end of official sec-support from php team
* supporting php 7.0 in v9 means people who upgraded to php 7.0 for a v8 upgrade don't have platform issues if upgrading to v9
* the php 7.1 language benefits are nice to have but no killer must-have features, we can live with the php 7.0 restrictions without crippling architecture too much
* we should better use v9 to further declare_strict=1 more classes, this needs huge efforts anyway, php 7.1 features are secondary and could be skipped for a core version
* supported in v7
* supported in v8
* supported in v9 - there is no php 7.2 yet, anyway.
php 7.2 is not on the horizon, but even if released:
* v7 already supports php 5.5->7.1, and if there is no killer breaking thing it 7.2, it may support that, too. -> v7 may evolve to support php 7.2
* v8 will hopefully run on php 7.2, better chances than v7. that would be in-line with typo3 core traditions, if there is no killer that gives us tons of headaches -> v8 will evolve.
Thus, I'd opt to support v9 with php 7.0, 7.1, 7.2 (if released), and whatever comes later in the LTS cycle of v9, if possible. For v10, we can then raise minimum PHP version to probably at least php 7.2, and will still have a good transition for people upgrading from v8 to v10, which would be perfect for our release cycles.
Sidenote: We can happily keep ourselfs busy with actually using more php 7.0 features in v9 for 1,5 years of development from now on, than jumping on the php 7.1 train now - which would only add additional upgrade issues to projects. For v9: Embrace php 7.0 features in the core. For v10: Raise php requirement as needed, can be discussed if time is ready.