If participating in employee-ownership ends up at the cost of a retiree having sufficient retirement savings -- such as if the employee can't or doesn't build a solid 401(k) or IRA -- that indeed seems risky.
DOL posts Form 5500 filings but they lag by 2-3 years: https://www.dol.gov/agencies/ebsa/employers-and-advisers/pla...
As of 2021 they listed 6,533 ESOP filings at a combined asset cap of about $2T. For context Apple's market cap just passed $3T in December.
Any mechanism which facilitates outside ownership erodes the effect of being ESOP.
It's an interesting thought, though. Effectively, assuming that the index were restricted to employees of member companies, setting up such a system would be tantamount to a corporate merger. In the event, if the failure of one member caused its employees to become ineligible to participate and thus autosell their shares, it could have a domino effect on other members. And if an outsider could hold shares in such an index, well, it's not really employee-owned anymore.