The short answer is: yes, you'll have to update your theme eventually - but you'll be able to upgrade to 2.7 without doing so, and will have the full lifecycle of the 2.7 release to upgrade your theme before we deprecate Bootstrap 2 and remove it from AtoM's code.
It's true that we have done work to upgrade AtoM to Bootstrap 5.1 - this will provide significant improvements in AtoM's responsiveness and accessibility. Additionally, there are several other legacy libraries and components we've managed to upgrade as part of this work, like the digital object carousel - ultimately this work will also improve AtoM's security and maintainability by removing a lot of deprecated code. Part of the reason that the 2.7 release has been delayed has been us trying to determine the best way to deliver these features without being too disruptive for existing users, and ensure we are providing resources to help users make the transition.
A lot has changed between Bootstrap 2 and Bootstrap 5, so yes, it's very likely that existing Bootstrap 2 themes will require revision - how much revision will likely depend on how complex the theme is. However, the current plan is that release 2.7 will be a transitional release - both Bootstrap 5 and Bootstrap 2 will be supported. This plan is still being developed, but for now our current intent is that in 2.7.0, our testing focus will be on the Bootstrap 2 version, to ensure we are delivering an excellent release. Anyone who wants to continue using the current look and feel - especially those with custom themes - will be able to do so for all 2.7.x releases.
The Bootstrap 5 theme and templates will be available in the release, so anyone with a custom theme (or anyone interested in developing a new BS5 theme, or just trying the new look and feel) can access the new templates and CSS in a development environment and have a good amount of time to upgrade any custom themes. However in 2.7.0, we won't focus as much on rigorous regression testing in Bootstrap 5 to avoid further delay. Instead, we will incrementally address any reports of issues or regressions particular to the BS5 templates, as well as fix any major bugs or regressions reported that affect the underlying code in both versions in patch releases. This will also give our team time to develop additional community resources, such as a theming upgrade guide. I expect that by 2.7.2 we will likely reach a place where we can call the BS5 part of the release as stable and production-ready as the BS2 templates, but this depends on the feedback we receive.
This is not to say that the BS5 templates and theme will be unusable in the 2.7.0 release. In fact, some of this upgrade work was done as a sponsored project, and you can already see our first Bootstrap 5 theme being used in production by the The United Nations Library & Archives Geneva:
So, despite the above, a good amount of testing has already gone into both versions, and we continue to gather feedback from our clients already using BS5 as a way of ensuring major regressions and issues have been addressed. Our hope is that some community users will choose to use the BS5 theme and templates sooner after the 2.7 release (either because they don't have a theme, or are prioritizing theme upgrades to take advantage of the improved accessibility and responsiveness), and that reports from those users will also help us identify and resolve any outstanding issues we may have missed during internal testing. If there are any public users who have access to experienced accessibility testers, we also hope to receive feedback on the accessibility improvements so we can further refine these as much as possible.
Another reason for the 2.7 release delay has been an overhaul of our translation platform. We've moved to a hosted Weblate solution, have upgraded Weblate to the latest version, and are currently overhauling our internal scripts (that handle moving strings from AtoM to Weblate and back) and translator documentation. Once this work is complete, we will reopen the AtoM Weblate instance for translations, which will be one of the final steps before preparing a public 2.7.0 release.
For now, all of the above is provisional, and details and release plans may still change. However, overall the plan is to provide a staged way to phase out Bootstrap 2 support, while giving our clients and community users time and supporting resources to help move to Bootstrap 5 before we remove the deprecated code in a future major release.