Apparently arch already applies an upstream patch for this.
However the patch is broken. You need
diff -Naur kdeconnect-kde-24.05.2/plugins/sftp/mounter.cpp kdeconnect-kde-24.05.2-manj/plugins/sftp/mounter.cpp
--- kdeconnect-kde-24.05.2/plugins/sftp/mounter.cpp 2024-06-28 23:08:24.000000000 +0200
+++ kdeconnect-kde-24.05.2-manj/plugins/sftp/mounter.cpp 2024-07-15 14:04:30.895329597 +0200
@@ -122,9 +122,9 @@
<< QStringLiteral("-o") << QStringLiteral("IdentityFile=") + KdeConnectConfig::instance().privateKeyPath()
<< QStringLiteral("-o") << QStringLiteral("StrictHostKeyChecking=no") // Do not ask for confirmation because it is not a known host
<< QStringLiteral("-o") << QStringLiteral("UserKnownHostsFile=/dev/null") // Prevent storing as a known host
- << QStringLiteral("-o") << QStringLiteral("HostKeyAlgorithms=+ssh-dss\\,ssh-rsa") // https://bugs.kde.org/show_bug.cgi?id=351725
- << QStringLiteral("-o") << QStringLiteral("PubkeyAcceptedKeyTypes=+ssh-rsa") // https://bugs.kde.org/show_bug.cgi?id=443155
- << QStringLiteral("-o") << QStringLiteral("uid=") + QString::number(getuid())
+ << QStringLiteral("-o") << QStringLiteral("HostKeyAlgorithms=ssh-rsa") // https://bugs.kde.org/show_bug.cgi?id=351725
+ << QStringLiteral("-o") << QStringLiteral("PubkeyAcceptedKeyTypes=+ssh-rsa") // https://bugs.kde.org/show_bug.cgi?id=443155
+ << QStringLiteral("-o") << QStringLiteral("uid=") + QString::number(getuid())
<< QStringLiteral("-o") << QStringLiteral("gid=") + QString::number(getgid())
<< QStringLiteral("-o") << QStringLiteral("reconnect")
<< QStringLiteral("-o") << QStringLiteral("ServerAliveInterval=30")
rather than the upstream patch 4f3a8968.patch