As a Salesforce consultant, there are additional implications of the upcoming new Salesforce UI.
The main one is this: whenever you build custom components, use the Salesforce Lightning Design System (SLDS) and other native elements.
When you use SLDS, you are guaranteed that any updates to Salesforce will be reflected in your components. So don’t use weird custom styling or icons, as they may look inappropriate with the UI refresh.
This applies to custom screen flows, Lightning Web Components, and Visualforce Pages. Elements include text, buttons, sections, columns, icons, and images.
I’m sure you’ve seen multiple managed packages that don’t follow SLDS, so hold onto your hats when the new UI arrives. Things could look ugly.
The takeaway
There’s a reason why one best practice is to use native over custom, and native styling over custom styling.