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Ken Koellner @ EngagewareKen Koellner @ Engageware 

Question on when Summer '21 hits the pre-release nodes

Question on when the next release hits the prerelease nodes.  I have a prerelease org I've been using for a while.  In Summer '21, the SLDS styles with "--" are supposed to be removed and the styles with "_" must be used instead.  I loaded a VF page in one of my orgs running on a prerelease node.  I see items like ".slds-tabs--default" in the sheet in addition to items like ".slds-tabs_default".  I would think that indicates either Summer '21 as not fully hit this prerelease node or that the change is not exactly as was documented; this change was announced for Summer '21 back in the Winter '21 release notes.
VinayVinay (Salesforce Developers) 
Hi Ken,

As per Summer'21 update notes,  Pre-Release Org’s will be released and available on the 15th April.  Check below reference for more details.

Hope this helps...