What's your question? Ask

Pruning now will affect next spring's blooms. If you look closely at the stems now, you'll see the start of the flower bud. I don't know if your plant has enough time to recover and form new buds. Where I live, plants are in the early stage of changing color. This indicates that plants are transitioning over to the stage of hardening off new growth before winter freeze hits. Of course, you can run your own experiment with a few of the wildest branches.

On some varieties, such as vining forsythia, it is normal for the foliage to turn dark. On the other hand, if it isn't a variety that normally turns dark, this can be a sign of slight over-fertilization. Now if you haven't fed the plant or changed its soil recently, then I would be inclined to believe that it is a pH issue. For controlling pH in soil, and just in general for the health of the soil zone, Dolomitic lime is a good thing to keep around. Just add it once or twice per year, and it will help. Here are a fews articles that will help: https://www.gardeningknowhow.com/ornamental/shrubs/forsythia/

Do you prune your forsythia annually; it blooms poorly on old wood. Remove 1/3 of the largest diameter central branches to the ground in spring after bloom time. Pruning after July 4th will drastically reduce next year's bloom. Forsythia also requires full sun for best bloom. The wild card is the weather. A warm spell in March followed by a return to freezing temps can ruin flower buds. Sporadic bloom after an early autumn cold snap is not uncommon.

It would be best to use potting soil for this. The soil may have too much clay in it. You can use perlite or vermiculite to fluff it up a bit. I would recommend using potting soil, though as this is made to drain well. Here is an article on the care of forsythia: https://www.gardeningknowhow.com/ornamental/shrubs/forsythia/forsythia-shrub-care.htm

Could you include a picture? Sometimes these can be normal, but other times it could signal an insect problem, or a fungal problem. If I am able to see it, then I can better help you find out what it is.