"The decision was based on ongoing reviews, client feedback and limited investor interest in the funds," according to an iShares letter to investors quoted by ETF.com. 

ETFs have been a successful innovation in the retail investment market. They are touted as inexpensive, well-diversified options for individual investors. 

To many analysts, target-date ETFs are a sensible solution to address both participant inertia, which some data suggests improperly balanced portfolios, and the question of high-fee investment options, which can absorb significant amounts of a participant's savings over the course of their working career. 

In a post on ETF.com, analyst Paul Britt called the funds' closures "a shame." 

"ETFs have been efficient, low-cost vehicles providing a complete long-term investment solution," wrote Britt. 

When packaged in a target-date fund, which automatically rebalances risk relative to a participant's age, ETFs served as the "epitome of set-and-forget vehicles," added Britt. "The investor can walk away for years and still have a balanced portfolio." 

But access to defined contribution plans has been made difficult for several reasons. 

For starters, ETF shares are divided into fractions, making them hard for institutions to work with. 

They are also more difficult than standard mutual funds to value on a day-to-day basis. 

But there's still hope for target-date ETFs in the 401(k) world. In February, Charles Schwab Retirement Plan Services announced the launch of a full-service 401(k) program based on ETFs. 

According to a Schwab press release, 401(k) plans that use index ETFs can reduce participant investment expenses by more than 90 percent compared to a 401(k) plan that primarily uses actively managed funds, and by more than 30 percent compared to a plan that uses index mutual funds.