Looking at the source code of Bootstrap v4.1, there are no utility classes included that specifically target the flex-basis value.

Searching through the file, there are only 12 occurances of the word flex-basis:

• 5 times to set flex-basis to 0 for some of the col-* classes (e.g. line 625).
• 7 times for styling specific navbar related classes (e.g. line 4268).

This also seem to remain the case in the latest versions of Bootstrap at the time of this writing:

• Bootstrap v4.5.3
• Bootstrap v5.0.0-alpha3

You do it just like you did. No library provides every functionality imaginable, especially when there's a variable that would make implementation very complex (such as an arbitrary size).

I do suggest a more semantic naming convention, though. Something like this aligns better with Bootstrap's model and best practice in general:

.flex-basis-30 {
  flex-basis: 30%;
}

.flex-basis-70 {
  flex-basis: 70%;
}

Also, use Bootstrap's classes for flexbox. No need to write your own. I've created a height class as well, in alignment with Bootstrap's sizing classes. However, it's often best to let your overall layout and content determine height rather than using fixed heights.

.h-100px {
  height: 100px;
}

<div class="d-flex flex-column h-100px">
  <div class="flex-basis-70">box</div>
  <div class="flex-basis-30">box</div>
</div>

I've built many Bootstrap sites over the last decade and a half, and every single one has had a custom stylesheet for a few things. Yours will too.