To some people, boutonnieres are out of style. To these people, boutonnieres do not seem necessary anymore. Hence, the thought that they are old-fashioned. But while that notion exists, you will still find boutonnieres at many weddings.
Men wear boutonnieres at a wedding for many reasons
As a fashion statement
As a sign of unity
As a sign of gratitude
If you cannot or do not want to wear a boutonniere, you can wear a lapel pin. A lapel pin goes in the same spot as a boutonniere. However, unlike boutonnieres, a lapel pin is easier to wear.
Boutonnieres are typically reserved for the groom, the wedding party, and any other person the couple wants to show some extra love or honor. But guests can also wear a boutonniere.
There is no consensus on the trendiness of boutonnieres. So, it is up to you whether to use a boutonniere or not.