We are on holidays, too. Also, we're all volunteers. Some things can be fixed quickly, some are not simple and/or not a priority. The OpenSky's foundation's stated mission is to support aviation research, so that will always be the main focus!
I am not saying that you're not deserving your holiodays.
But there are hundreds of other Forums out in the wide Internet where the owners left already and leaving it alone. That's a feeling which can grow quickly.
I am working in customer support as well. Nobody is expecting a solution on first hand. But leaving clients uninformed is never a good idea.
You are on vacation? Good, let us know! put a banner in the Forum header saying that it takes longer, put an announcement thread in the general section and so on.
You're already working on a reported bug? Good, then simply tell us that you are.
But if a support request is fully unanswered, then such impressions can grow.
To be fair, there were only a couple of blog posts around that time, none before or after. I just didn't have the time or anything particularly important to blog there, Twitter does a good job for most things really. The correlation of the blog with the activity level of the OpenSky team is certainly non-existent but that's not a good reason to get rid of the blog, the existing posts are fairly popular.
I guess we should try to give an impression of active development to the outside world but we rather focus on doing things where possible than communicating them here. Even so, there've been some major additions such as FLARM recently (see opensky-network.org/forum/feature-update...-data-to-opensky#836), a TCAS-related decoder (www.cs.ox.ac.uk/files/11077/DASC_2019.pdf) and other things which are planned and about which we will talk when they are closer to release. It is also very much not advisable to talk about plans that may or may never come to fruition. ;)
Some other features have been tried and tested and found to be lacking (say, live tracks as opposed to flights), we normally communicate these issues on Twitter, which contrary to your assertion has been way more active in the past 12 months than ever before.
Some clear indications of activities are always and currently on the front page though. We've been running a competition this year (more to come on that, too, at some point). There is a tutorial happening at the major avionics conference about the use of OpenSky data for cool stuff (slides will hopefully bee made available after) and we are running yearly workshops for the community, where everyone is welcome. It's been growing massively over the past few years, there's so much more interest on all levels!
Finally, OpenSky is a non-profit foundation. All other existing websites/trackers are doing super great things, which we really appreciate, but are of course all limited companies of some sort. This makes for a completely different approach and direction.
Giving the clients which are supporting your network by feeding the data the feeling that nobody takes care is always a bad option.
Clients which left the service getting back is a "mission impossible".
So it's good that you take the opportunity letting us (the clients) know that there is still activity behind the scenes.
I am working in IT support of a large company and understand how difficult it is. But we made since several years the experience to tell the clients more what you're working on.
Once they start asking, it is almost too late.