<apex:slds /> you are always receiving the latest version of CSS from SLDS while using the lightningStylesheets="true" does not add the SLDS stylesheets to the page, but instead changes the standard stylesheets from Visualforce to "look" like SLDS.
when using lightningStylesheets="true", Observing the network tab traffic, we clearly see that SLDS is not loaded, but instead a mock slds:
VS using the apex:slds Tag
Style Existing Visualforce Pages with Lightning Experience Stylesheets
Some components differ slightly in style from Lightning Experience.
<apex:inputFile>, and some elements use the
browser’s default styling instead.
To include SLDS components that aren’t part of the Visualforce
component library, you can use the
<apex:slds /> tag with custom SLDS
code in addition to the lightningStylesheets attribute.
ThelightningStylesheets attribute doesn’t affect custom styling, so
custom code and must be updated to match the page’s SLDS styling.
lightningStylesheets="true" does not deprecate apex:slds
they can even co-exist together in the same page.
The link above contains the list of Visualforce components that support the lightningStylesheets attribute or don’t require styling.