What exactly does this mean

I don't know. That's hidden in the darkness of undocumented improvements that aren't used yet in Joomla 4 ;-)

what do I do to get rid of this warning?

All I know is that you can use Joomla\CMS\HTML\Helpers\Select::option(8800, 'Bulletin Board'); but is that what we shall use after reading use the service registry instead?

That means thing like JHTML::_('segment1.segment2.segment3') is deprecated.

I think you got warning from somewhere else, not from JHTML::_('select.option', 8800, 'Bulletin Board'), it have 2 segments.

it have 2 segments.

No.

JHTML::_('select.option', 8800, 'Bulletin Board')

or

HTMLHelper::_('select.option', 8800, 'Bulletin Board')

have 3 segments/parts at the end where the deprecated message is thrown:

jhtml
select
option

https://github.com/joomla/joomla-cms/blob/4.2-dev/libraries/src/HTML/HTMLHelper.php#L89-L94

• Add line HTMLHelper::_('select.option', '0', 'JDISABLED'); in Cassiopeia index.php.
• Debug code place above with a print_r or whatever.

Several HTMLHelper calls will appear with 3 segments.

Ah okay, I see, it called second tyme by HTMLHelper::register() with modified key.

@skurvish you should not worry about this message, it not a bug, just a notification about future changes.
Just set error reporting to exclude Deprecation messages.
Thanks.

Use of Joomla\CMS\HTML\HTMLHelper::_('select.option', 8800, 'Bulletin Board') is safe.