Behind every statement in every company claiming to prioritize employees' development and personal journeys, is a tension lurking in the system.
The tension is that for many managers, the right choice for the journey of an employee is sometimes the wrong choice for the project they're working on.
I've seen this happen many times (with the best of intentions). The internal managerial conversations go something like this...
"Gee, I'd like to allow for learning time, but I can't justify the impact to the project and what happens to me if it's late.."
"That person would like to learn React Native, but I need PHP developers more right now..."
Because employees watch what the organization does, not what the organization says in Town Hall meetings, the tension builds until suddenly, vaguely-worded exit interview responses with words like "opportunities" and "next steps in my career" appear.
If your startup is really interested about retaining people for the long term, there are two ways to approach it.
The Coach - recognize that another role is needed. A coach, not accountable to the delivery of work, able to serve as an advocate and developmental champion of a number of employees. They can structure developmental offerings, push for new roles, and add a distinct practice to your organization.
The Culture - make advancement and growth part of your culture, to the point that taking risks on people and pushing development and honest feedback above all is actually an asset to your business and inseparable from your deliveries. I cited it earlier, but "An Everyone Culture" by Robert Kegan and Lisa Laskow Lahey is an incredible guide to this dynamic.