As Beda answered on https://toolset.com/forums/topic/pull-out-data-from-toolset/page/2/#post-1080228, I will keep here, where it should to be.
1) Right, I noticed that Views Loop options for Grid are disabled if Bootstrap is disabled in Settings. I have some ideas how to handle it manually. However, if that is unique 'critical' (dependent) usage, I can't see the issue as View Loop Grid in any case use EXCLUSIVELY ".col-sm-$" class and it makes ZERO impact on Bootstrap 3 and 4 (as only view-ports/breakpoints are different > now is 576px vs previous 768px > or sm is now md).
"WORKAROUNDS" (and some lack of the Toolset consistency)
So, vs. Views Loop Grid, Toolset in any case use only ".col-sm-$" class, what makes it inconsistent with Bootstrap 'mobile first' approach, as it excluded applicability of view-ports combinations (ie - <div class="col-sm-6 col-lg-3">).
Every View (if needed), should be in any case manually "fixed" even now, what actually should not to be so easy to do (view-ports should be combined). Bootstrap 3 to 4 'fix' is much more easier to 'fix', as only ".col-sm-$" class should be replaced with ".col-md-$" class. Equally, it makes it relatively easy also to replace with other Grid systems (ie - for Genesis could be replaced with > "one-fourth", but it will not work "out of the box" vs missing solution for class "first"), like Skeleton (instead of <div class="col-sm-3">, should be <div class="one-fourth columns">).
As we can see, frameworks is less important, lack of consistency by Toolset is the real issue, whatever framework will be used, as Views Loop Grid is actually not 'mobile friendly', despite Bootstrap usage.
Present situation in fact imply usage of "Unformatted" Views Loop (so, as Bootstrap is disabled) IN ANY OCCASION and than manually "fix' of such 'loop', depending of view-ports (what is not so easy, especially as depends on what Grid system will be used).
IMPROVEMENT SUGGESTION (this is not feature request, as I explained how here is about lack of inconsistency)
Bootstrap 4 is easy to support only by enable in dialog option to manually insert class (sm or md). However, significant issue is lack for view-ports support.
2) I don't use Layouts in any case.
3) To repeat question > Is it some other ("hidden", unknown, ...) Toolset dependency on Bootstrap 3?
P.S. I guess that there is no such dependency, as in Settings is option to not be used Bootstrap (at all), but logically I must to ask this question, as there is many pages on Toolset where is clearly stated how Toolset is grounded on Bootstrap (version 3).